Support astgrep (#4975)

* astgrep: support its LSP

Fixes: #4974

* astgrep: add a fixer

test/fixers/test_astgrep_fixer_callback.vader

@@ -0,0 +1,27 @@
+Before:
+  Save g:ale_astgrep_executable
+  Save g:ale_astgrep_scan_options
+
+After:
+  Restore
+
+Execute(The ast-grep callback should return the correct default values):
+  AssertEqual
+  \ {
+  \   'command': ale#Escape('ast-grep') . ' scan --update-all %t',
+  \   'read_temporary_file': 1,
+  \ },
+  \ ale#fixers#astgrep#Fix(bufnr(''))
+
+Execute(The astgrep executable and options should be configurable):
+  let g:ale_astgrep_executable = '/path/to/ast-grep'
+  let g:ale_astgrep_scan_options = '--interactive'
+
+  AssertEqual
+  \ {
+  \   'command': ale#Escape('/path/to/ast-grep')
+  \     . ' scan'
+  \     . ' --interactive %t',
+  \   'read_temporary_file': 1,
+  \ },
+  \ ale#fixers#astgrep#Fix(bufnr(''))